This invention relates generally to electrostatic imaging and more particularly to the conditioning of powder images for rendering them insensitive to development systems through which they are transported.
In the process of electrostatic imaging such as plain paper xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, as by heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky while simultaneously applying pressure. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
With the advent of processors capable of creating multiple images using corona emissions, toner image disturbance of an already developed powder image passing through one or more developer housings had to be address. Early attempts at obviating such image disturbances led to the use of scavengeless developer systems. One such system utilizes biased electrodes to form toner clouding in the development zone intermediate the imaging or charge retentive surface and a developer housing. In an Image On Image (IOI) xerographic system using scavengeless development, the forgoing problem of image disturbance is dealt with using a split charging system such as disclosed in U.S. patent application Ser. No. 08/347,617 filed on Nov. 30, 1994 in the name of Folkins et al. The purpose of using a split recharge system as disclosed in the aforementioned application is to insure that developed and non-developed image areas of a charged retentive surface are at or near the same electrical potential when passing through a developer housing. However, such development systems tend to damage images through a phenomena know as Under Color Splatter (UCS). UCS is caused when a neutralized or wrong sign toner layer passes through toner clouds of a scavengeless developer housing through which it passes. The toner cloud knocks loosely held toner particles off developed images causing a halo on the trail edge of the new image.
The Recharge/Expose/and Develop (REaD) imaging process as disclosed in the aforementioned patent application is repeated for each subsequently developed image in superimposed registration on the charge retentive surface until a complete image, in the case of the '617 patent application, a full color image is created. The different colors may be developed on the photoreceptor in an image on image development process, or a highlight color image development process (image next-to image). As may be appreciated, the REaD profess is not restricted to color imaging. For example, it may also be used in a process where only black images are created with clear toner being used for forming glossy images.
The images may be formed by using a single exposure device, e.g. ROS, where each subsequent color image is formed in a subsequent pass of the photoreceptor (multiple pass). Alternatively, each different color image may be formed by multiple exposure devices corresponding to each different color image, during a single revolution of the photoreceptor (single pass). As will be appreciated, various other combinations are possible.
Several issues arise that are unique to the REaD IOI process of creating multi-color images in the attempt to provide optimum conditions for the development of subsequent color images onto previously developed color images. For example, during a recharge step, it is important to level the voltages among previously toned and untoned areas of the photoreceptor so that subsequent exposure and development steps are effected across a uniformly charged surface. The greater the difference in voltage between those image areas of the photoreceptor previously subjected to a development and recharge step; those image areas subjected to a development step, but not yet subjected to a recharge step; and those bare non-developed, untoned areas of the charge retentive surface, the larger will be the difference in the development potential between these areas for the subsequent development of image layers thereon.
Another issue that must be addressed with the REaD IOI color image formation process is the residual charge and the resultant voltage drop that exists across the toner layer of a previously developed area of the photoreceptor. Although it may be possible to achieve voltage uniformity by recharging this previously toned layer to the same voltage level as neighboring bare areas, the associated residual toner voltage (V.sub.r) prevents the effective voltage above any previously developed toned areas from being re-exposed and discharged to the same level as neighboring bare photoreceptor areas which have been exposed and discharged to the actual desired voltage levels. Furthermore, the residual voltage associated with previously developed toner images reduces the dielectric and effective development field in the toned areas, thereby hindering the attempt to achieve a desired uniform consistency of the developed mass of subsequent toner images. The problems become increasingly severe as additional color images are subsequently exposed and developed thereon. Color quality is severely compromised by the presence of the toner charge and the resultant voltage drop across the toner layer. The change in voltage due to the toned image can be responsible for color shifts, increased moire, increased color shift sensitivity to image misregistration and motion quality, toner spreading at image edges, and loss in latitude affecting many of the photoreceptor subsystems. Thus, it is ideal to reduce or eliminate the residual toner voltage of any previously developed toned images.
Prior attempts to address one or more of these issues have introduced a variety of secondary problems, each having an adverse effect on the image on image color image formation process. For example, the concurrently filed, copending application for patent entitled "Method and Apparatus for Reducing Residual Toner Voltage", Ser. No. 08/347,616, by a common assignee as the present application, discloses a voltage sensitive recharge device used for the recharging steps during a color image formation, whose graph of the output current (I) to the charge retentive surface as a function of the voltage to the charge retentive surface (V) has a high (I/V) slope. The high I/V slope recharge device disclosed having an AC voltage supplied thereto, enables an extended time for neutralization to occur at the top of the toner layers. However, the amount of residual voltage V.sub.r reduction that can be realized is somewhat limited.
Another recharging method is described in application for Japanese Patent No. Hei 1-340663, Application date Dec. 29, 1989, Publication date Sep. 4, 1991, assigned to Matsushita Denki Sangyo K.K. This reference discloses a color image forming apparatus wherein a first and second charging device are used to recharge a photoconductor carrying a first developed image, before exposure and development of a subsequent image thereon. The potential of the photoconductor is higher after passing the first charging device than after passing the second charging device. This reference teaches that the difference in voltage applied by the first and second charging devices to the toner image and photoreceptor surface is set to a relatively high level, to insure that the polarity of the toner image is reversed after passing and having been charged by both devices. The effect of this teaching is to reduce the residual charge in the image areas which becomes more severe when applying color toners onto previously developed color toners, and also to prevent toner spray (or toner spread) during the exposure process. Toner spray is a phenomena caused when the photoconductor carrying the first toner image is recharged to a relatively high charge level and then exposed for the second image development. In areas where the edges of a prior developed image align but do not overlap with the edges of a subsequent image, the toner of the prior image tends to spray or spread along its edges into the subsequently exposed areas which have a relatively lower charge level. By reversing the polarity of the toner as taught in this reference, toner spray is prevented, as the reversed polarity toner is no longer attracted to the exposed areas.
However, when a substantial amount of toner charge at the top of a previously developed toner layer is reversed in polarity during recharge, a different problem of a serious nature develops. Since the prior toner image is now predominantly of an opposite polarity to both the bare background areas and the incoming color toner to be developed thereon, an interaction occurs among these three separate and distinctly charged regions. For example, in a system having a negatively charged photoreceptor using Discharged Area Development (DAD), the negatively charged toner used for development would be reversed in polarity after recharge using the teachings of Matsushita. Particularly, the now-positively charged toner layer is then attracted to the negatively charged background areas and the negatively charged toner of the incoming color image. Thus, the positively charged toner of the first image tends to splatter into neighboring bare background regions. This occurrence has been titled the Under Color Splatter (UCS) defect and is the cause of unwanted blending of colors and the spreading of colors from image edges into background areas. The UCS defect is apparent both where the prior image aligns with a subsequent image, and also where the prior image overlaps with the subsequent image. Consequently, color clarity is severely impacted. Furthermore, when a relatively large voltage difference between the first and second charging devices is applied to the photoreceptor surface in order to reverse the polarity of the toner image, a significant amount of stress is applied to the photoreceptor, which may also negatively impact image quality, as well as reduce the life expectancy of the photoreceptor.
Notwithstanding the efforts that have been made as described above, UCS in scavengeless development systems has persisted.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, should provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 5,282,006 relates to an apparatus for transferring a developed toner image from an image bearing surface to a final support substrate including a corona generating device for establishing a transfer field and a pressure treatment apparatus for compacting the toner image on the image bearing surface. The pressure treatment apparatus substantially prevents premature transfer of toner across air gaps between the image bearing surface and the final support substrate.
U.S. Pat. No. 4,833,503 discloses a multi-color printer wherein a recharging step is employed following the development of a first image. This recharging step, according to the patent is used to enhance uniformity of the photoreceptor potential, i.e. neutralize the potential of the previous image.
U.S. Pat. No. 4,660,059 discloses an ionographic printer. A first ion imaging device forms a first image on the charge retentive surface which is developed using toner particles. The charge pattern forming the developed image is neutralized prior to the formation of a second ion image by a corona generating unit and an erase lamp.
U.S. Pat. No. 5,208,636, discloses a printing system wherein charged area images and discharged area images are created, the former being formed first and the latter being proceeded by a recharging of the imaging surface.
U.S. Pat No. 5,241,356 discloses a multi-color printer wherein charged area images and discharged area images are created, the former being formed first, followed by an erase step and a recharge step before the latter is formed. An erase lamp is used during the erase step to reduce voltage non-uniformity between toned and untoned areas on a charge retentive surface.
U.S. Pat. No. 5,258,820 discloses a multi-color printer wherein charged area images and discharged area images are created. An erase lamp is used following development using Charged Area Development (CAD), and a pre-recharge corona device is used following development using Discharged Area Development (DAD) and prior to a recharge step, to reduce voltage non-uniformity between toned and untoned images on a charge retentive surface.
Application for U.S. patent titled "Method and Apparatus for Reducing Transferred Background Toner", Ser. No. 08/346,708 by a common assignee as the present application, discloses a corona recharge device for recharging the photoreceptor containing at least one previously developed color image, to a voltage level intermediate to the background areas and the image areas. This intermediate recharge level keeps wrong-charge toner developed in the background areas at a charge level distinct from the toner developed in the image areas, so that the wrong-charge background toner does not transfer to a support substrate with the image.
A number of commercial printers employ the REaD imaging process. For example, the Konica 9028, a multi-pass color printer forms a single color image for each pass. Each such pass utilizes a recharge step following development of each color image. The Panasonic FPC1 machine, like the Konica machine is a multi-pass color device. In addition to a recharge step the FPC1 machine employs an AC corona discharge device prior to recharge.